The OpenCL functions that are submitted to a command-queue are enqueued in the order the calls are made but can be configured to execute in-order or out-of-order.
The properties argument in clCreateCommandQueueWithProperties can be used to specify the execution order.
If the CL_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE property of a command-queue is not set, the commands enqueued to a command-queue execute in order.
For example, if an application calls clEnqueueNDRangeKernel to execute kernel A followed by a clEnqueueNDRangeKernel to execute kernel B, the application can assume that kernel A finishes first and then kernel B is executed.
If the memory objects output by kernel A are inputs to kernel B then kernel B will see the correct data in memory objects produced by execution of kernel A.
If the CL_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE property of a command-queue is set, then there is no guarantee that kernel A will finish before kernel B starts execution.
Applications can configure the commands enqueued to a command-queue to execute out-of-order by setting the CL_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE property of the command-queue.
This can be specified when the command-queue is created.
In out-of-order execution mode there is no guarantee that the enqueued commands will finish execution in the order they were queued.
As there is no guarantee that kernels will be executed in order, i.e.
based on when the clEnqueueNDRangeKernel calls are made within a command-queue, it is therefore possible that an earlier clEnqueueNDRangeKernel call to execute kernel A identified by event A may execute and/or finish later than a clEnqueueNDRangeKernel call to execute kernel B which was called by the application at a later point in time.
To guarantee a specific order of execution of kernels, a wait on a particular event (in this case event A) can be used.
The wait for event A can be specified in the event_wait_list argument to clEnqueueNDRangeKernel for kernel B.
In addition, a wait for events (clEnqueueMarkerWithWaitList) or a barrier (clEnqueueBarrierWithWaitList) command can be enqueued to the command-queue.
The wait for events command ensures that previously enqueued commands identified by the list of events to wait for have finished before the next batch of commands is executed.
The barrier command ensures that all previously enqueued commands in a command-queue have finished execution before the next batch of commands is executed.
Similarly, commands to read, write, copy or map memory objects that are enqueued after clEnqueueNDRangeKernel or clEnqueueNativeKernel commands are not guaranteed to wait for kernels scheduled for execution to have completed (if the CL_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE property is set).
To ensure correct ordering of commands, the event object returned by clEnqueueNDRangeKernel or clEnqueueNativeKernel can be used to enqueue a wait for event or a barrier command can be enqueued that must complete before reads or writes to the memory object(s) occur.